1. Field of the Invention
The present invention relates to a liquid crystal display device using plastic substrates which is principally used as a display device for Office Automatic (OA) appliances such as a note type personal computer, a word processor, and a palm-top type personal computer.
2. Description of the Related Art
A typical liquid crystal display device is comprised of a pair of plastic substrates, transparent electrodes, insulating films for protecting the electrodes, aligning or orientation films and a liquid crystal.
The insulating film (top coat) is applied by overcoat processing and takes the form of a thin protective layer of Si oxide or Ti oxide on the surface of the electrode formed on the substrate.
Most of these type of liquid crystal display devices are manufactured, as shown in FIG. 1, by preparing two glass substrates 11, providing on the inner surface of each glass substrate 11 a lower protective film (SiO.sub.2 film) 12, an ITO (indium oxide added with tin) electrode 13, an insulating film 14 and an aligning film (polyimide film) 15, opposing the two glass substrates to each other and filling a gap therebetween (cell gap) with a liquid crystal 17. Denoted by 10 is a polarizer plate and by 16 is a seal agent. The lower protective film 12 prevents impurities (Na and the like) in the glass substrate 11 from migrating into the liquid crystal 17. On the other hand, when conductive foreign matters having a size comparable to the cell gap mix in the liquid crystal layer 17, the insulating film (top coat) 14 protects the ITO electrode 13 electrically and mechanically to prevent faulty leakage.
In the past, solution for formation of the insulating film 14 is prepared by dissolving a prepolymer (or monomer) of Si (or Si and Ti) added with hydroxyl group--OH or alkoxy group--OR in a solvent of high boiling point such as normal methyl pyrrolidone or dimethyl acetamide at a percentage of 3 to 10 wt % of solid components. The solution will hereinafter be called "silica coating ink" or simply referred to as "ink" and is commercially available as, for example, a MOS,Si film, or Ti--Si film manufactured by Tokyo Ohka Kogyo (Japanese corporation). Thus, the insulating film is formed by printing the silica coating ink on the ITO electrode 13 by offset printing technique and thereafter sintering the print at a temperature of 250.degree. to 350.degree. C. More particularly, the silica coating ink is subjected to dehydration condensation to create --Si--O--Si-- or --Ti--O--Ti-- (for example, through a reaction from silanol to siloxane) to form an oxide of the ink. Through this, the insulating film 14 is provided with insulating capability and hardness necessary for the insulating film to function as a so-called top coat.
To effect the dehydration condensation reaction completely, a temperature of 500.degree. C. is needed but for the insulating film (top coat) 14 of the liquid crystal display device, a sintering temperature of 250.degree. C. is sufficient.
Further, in order to reduce the size and thickness of the liquid crystal display device and to increase its impact-proof capability, it is desirable to use plastic film substrates (made of, for example, polyether sulfone (PES), polyethylene terephthalate (PET), aryl diglycol carbonate (ADC) or acrylic resin in place of the glass substrates 11. In this case, thermal stability of the plastic film substrate is ensured at 200.degree. C. or less and correspondingly the aforementioned sintering temperature must be limited to 200.degree. C. or less. However, by simply setting the sintering temperature to 200.degree. C. or less, the aforementioned dehydration condensation reaction does not proceed satisfactorily, leaving behind the hydroxyl group--OH alkoxy group--OR and a solvent in the top coat 14. As a result, there arises a problem that the insulating capability and hardness for protecting the ITO electrode 13 cannot be ensured.
To cope with this problem, the present applicant has already proposed Japanese Patent Application No. Hei 3-71476, according to which in order to provide a liquid crystal display device having an upper insulating film capable of being formed at a sintering temperature of 200.degree. C. or less without thermally changing the nature of a plastic film substrate and capable of having sufficient insulating capability and hardness for protecting an ITO electrode, a top coat is used which is formed through the steps of coating silica coating ink as a material of the top coat on the surface of the electrode formed on the substrate, irradiating ultraviolet light (UV light) on the silica coating ink, and sintering the silica coating ink having experienced the ultraviolet light irradiation at a sintering temperature of 200.degree. C. or less.
Thus, conventionally, when etching and patterning the transparent electrode and depositing a silica coating material (for example, MOF Ti--Si film manufactured by Tokyo Ohka Kogyo (Japanese corporation))on the etched and patterned transparent electrode in a thickness of 700 to 1400 .ANG. through print process to form an insulating layer having hardness and insulating capability, the above technique is applied by irradiating UV light on the coating material by means of a low pressure mercury vapor lamp to volatilize a solvent in the coating material, with a view of permitting low temperature sintering at about 200.degree. C.
The problem resides in the fact that the silica coating film should be sintered at 250.degree. C. or more to have necessary hardness and insulation for the top coat (insulating film) but at that temperature the thermal stability of the plastic substrate is degraded. Therefore, in effect, the heating temperature is set to about 200.degree. C. allowable for the thermal stability and a solvent in the coating material is volatilized under the irradiation of UV light by a low pressure mercury vapor lamp to form a topcoat film (insulating film).
However, the UV light on the one hand can contribute to achievement of low temperature sintering of the top coat (insulating film) but on the other hand can destroy the plastic substrate, and besides a resulting silica coating film is so hard that it may crack disadvantageously when the plastic substrate deforms.